To change a folder icon on Windows, right-click on the folder that is being changed and select properties. Then, from the 'Customize' tab, press 'Change Icon.' This opens a dialog that allows the user to choose a preloaded folder icon or browse for an image.
If using a Mac, the process is similar. On OS X, copy the desired image to the clipboard. Then, select the folder to be changed and press Cmd + i to open the folder's info panel. Click on the icon in the top left corner of this panel and press Cmd + v to paste the new icon. OS X also supports drag-and-drop, and this allows an image from a browser or file to load into the folder's info panel in order to change the icon. Some image formats do not work correctly when used this way however, and in these cases the image or file must be copied and pasted.
Windows offers another option for the tech savvy as well. Change the icon of a folder by opening (or creating if it doesn't exist) the hidden 'desktop.ini' file within that folder. The file has this structure:
[.ShellClassInfo] IconFile=(file path for the icon image) IconIndex=20
The icon can be manually set by changing the file path after 'IconFile' to the path of the desired icon. If the image is an .ico file, change the IconIndex to '0'.